[Show abstract][Hide abstract] ABSTRACT: Anomalous and nonanomalous surface X-ray diffraction is used to investigate the atomic structure and composition of the yttria-stabilized zirconia (YSZ)(111) surface. By simulation it is shown that the method is sensitive to Y surface segregation, but that the data must contain high enough Fourier components in order to distinguish between different models describing Y/Zr disorder. Data were collected at room temperature after two different annealing procedures. First by applying oxidative conditions at 10(- 5) mbar O2 and 700 K to the as-received samples, where we find that about 30% of the surface is covered by oxide islands, which are depleted in Y as compared with the bulk. After annealing in ultrahigh vacuum at 1270 K the island morphology of the surface remains unchanged but the islands and the first near surface layer get significantly enriched in Y. Furthermore, the observation of Zr and oxygen vacancies implies the formation of a porous surface region. Our findings have important implications for the use of YSZ as solid oxide fuel cell electrode material where yttrium atoms and zirconium vacancies can act as reactive centers, as well as for the use of YSZ as substrate material for thin film and nanoparticle growth where defects control the nucleation process.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the optical properties of self-assembled diindenoperylene nanocrystals grown epitaxially on a copper hexadecafluorophthalocyanine film by SNOM imaging and near-field photoluminescence. The coupled SNOM-spectrometer system allows us to study the optical response of a single nanocrystal beyond the diffraction limit. Our results demonstrate the existence of a diindenoperylene wetting layer connecting the nanocrystals.
[Show abstract][Hide abstract] ABSTRACT: The rapidly increasing power conversion efficiencies of organic solar cells are an important prerequisite towards low cost photovoltaic fabricated in high throughput. In this work we suggest indane as a non-halogenated replacement for the commonly used halogenated solvent o-dichlorobenzene. Indane was blended with the higher volatile solvents chloroform or toluene or o-xylene in order to improve wettability and to reduce drying time. The combination of high volatile solvents with the less volatile host solvent indane allows for an increased fabrication speed due to a reduction of the overall drying time and provides films with good light absorption behavior and high polymer crystallinity. For the solvent mixture toluene-indane, solar cell performance is comparable to the o-dichlorobenzene reference device indicating this mixture as a suitable replacement for increased productivity without drawbacks in nanomorphology as investigated by atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXD). This study provides a fundamental understanding on solvent mixture drying kinetics and can aid the ink formulation. (C) 2011 Elsevier B.V. All rights reserved.
Solar Energy Materials and Solar Cells 01/2012; 96(1):195-201. · 4.63 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied the interaction of oxygen with MgO(100) supported Pd nanoparticles at 10(-5) mbar oxygen pressure and a sample temperature of 570 K. We employed high-resolution X-ray reciprocal space mapping, which allows us to resolve the average particle shape from the quantitative analysis of intensity diffraction rods running perpendicular to corresponding facet surfaces. We identified the oxygen induced formation of nanosized (112) facets which is reversible in a CO atmosphere. Our results give direct evidence for the microscopic evolution of the nanoparticle shape under reactant exposure, which is essential for an atomistic understanding of catalytic reactions on nanoparticles.
[Show abstract][Hide abstract] ABSTRACT: Organic bulk heterojunction (BHJ) solar cells comprising con-jugated polymers (as electron donor) and fullerene derivatives (as electron acceptor) deposited from solution make low cost photovoltaic-energy conversion feasible. [ 1 , 2 ] The active layer of BHJ solar cells comprises an interpenetrating network of polymer and fullerene domains that forms during deposition and drying. The energy-conversion process involves the crea-tion of excitons upon light absorption which are dissociated into free charges at the donor/acceptor interface and are further transported towards the respective electrodes. Thus, for a given combination of donor and acceptor materials, the effi ciency of a BHJ critically depends on its nanoscale structural properties. Ideally, BHJs must have a large interface with domain sizes comparable to the exciton diffusion length (a few nano meters) and effective percolation paths to the electrodes, with balanced transport of holes and electrons. In reality, because of the variety and complex interplay of thermodynamic and kinetic factors that infl uence the resulting nanomorphology, optimiza-tion of the microstructure and the morphology is a laborious task that involves trialling many processing protocols. [ 3–5 ] Over recent years numerous studies have been performed on the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) with the aim of advancing the understanding of the structure–property–performance relation-ships. The development of P3HT crystalline order with a π – π stacking of the molecules has been found to be an important factor in ensuring suffi cient hole-carrier mobility for charge extraction. [ 6–8 ] Other structural issues of importance are the rel-ative orientation of P3HT crystallites (due to their anisotropic electrical charge mobility) and the distribution of amorphous P3HT regions (which are expected to affect the electronic inter-connection among P3HT domains). [ 6 , 9 ] However, the relative importance of the many implied microstructural features on solar-cell performance and the factors determining the develop-ment of specifi c structures during blend solidifi cation are still poorly understood. As many results of studies into the effect of processing conditions on nanomorphology cannot be trans-ferred to an industrially relevant process (because the coating shear forces and solvent-evaporation conditions are different), we have chosen doctor-blading as a lab-scale fabrication method which is scalable to roll-to-roll (R2R) processing. [ 10 , 11 ] Here, the effect of substrate temperature on the structural evolution of P3HT:PCBM blends has been investigated using in situ grazing incidence X-ray scattering (GIXS). In situ X-ray meas-urements allow real–time observation of the emergence and evolution of the blend microstructure during solvent evapora-tion. The characterization of the dried fi lms is complemented with ex situ X-ray measurements, optical-absorption data, and the application of atomic force microscopy (AFM) to explore the degree of P3HT:PCBM phase segregation upon removal of the PCBM component from the fi lm. This work provides fundamental understanding of the polymer: fullerene micro-structure evolution during drying. We further demonstrate that lowering the substrate temperature during coating and drying offers a control parameter to achieve a more optimized blend nanomorphology. For the in situ study of the structural evolution during drying, a solution of P3HT and PCBM dissolved in dichlorobenzene (DCB) was doctor–bladed on poly(3,4–ethylenedioxythiophen e):poly(styrenesulfonate) (PEDOT:PSS) coated glass substrates at substrate temperatures of 10 ° C, 25 ° C, 40 ° C, and 80 ° C. Immediately after coating, real–time X-ray scattering data was collected with a two-dimensional (2D) Mar–CCD (charge-cou-pled device) camera (Figure 1). An optical refl ectometer allowed simultaneous measurement of the blend–solvent fi lm thickness
[Show abstract][Hide abstract] ABSTRACT: An understanding of the structure of ultrathin polymer films on solid substrates has scientific importance in applications as well as in fundamental studies of polymer diffusion or adsorption. We present studies of the organization of dewetted droplets of polymers on a silicon surface using a new neutron scattering technique, spin-echo resolved grazing incidence scattering (SERGIS), that has the potential to address at the same time the droplet-droplet correlations and the chemical configuration inside each droplet. For the seminal experiments, the polarized neutron reflectometer EVA at the Institut Laue-Langevin, Grenoble, France, was equipped with a spin-echo setup, and measurements were taken on surface structures previously characterized by different techniques. The dewetted polymers used in our studies were pure polystyrene, a mixture of polystyrene and polyparamethylstyrene, and a diblock copolymer of the two homopolymers. Even with a provisional setup SERGIS, we were able to determine the correlation between the droplets, providing results in excellent agreement with those obtained by atomic force microscopy and grazing incidence small-angle X-ray and neutron scattering. In addition, it was confirmed that the correlation function for diblock copolymer droplets is more complex than for polymer mixtures, exhibiting partial ordering of the copolymer within each droplet.
The Journal of Physical Chemistry B 05/2011; 115(19):5754-65. · 3.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a route to change the "compositional" order of highly crystalline binary layers comprising diindenoperylene and copper-phthalocyanines from two- to one-dimensional periodicity. This is achieved by exchanging fluorine with hydrogen atoms in the phthalocyanines, thereby reducing the C-F···H-C interactions and allowing the interplay of long-range electrostatic interactions in mesoscopic phases. Linear patterns are thus obtained, whose periodicity can be additionally tuned by an appropriate stoichiometry of the components.
Physical Chemistry Chemical Physics 03/2011; 13(10):4220-3. · 3.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We combine X-ray reflectivity and scanning electron microscopy measurements to investigate the mechanisms involved in the growth of vertical arrays of phthalocyanine nanowires directed by templates of Au nanoparticles. The study has been carried out for H(16)CuPc at different substrate temperatures. It is shown that three organic morphologies evolve during the growth: 1D nanostructures on top of the Au nanoparticles, a multilayer film on the substrate and a layer wetting the gold nanoparticles. For substrate temperatures below 100 °C there is a coexisting and competing growth of the three structures, whereas beyond this temperature the 1D growth on the nanoparticles is predominantly favored. The observance of two regimes with the temperature is characterized by two different activation energies. Both the length of the 1D structures and the thickness of the multilayer film can be precisely controlled by the substrate temperature which is of importance for application of vertical organic nanowires as donor/acceptor architecture in organic solar cells.
Physical Chemistry Chemical Physics 02/2011; 13(13):5940-4. · 3.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Interfaces are inherent in and essential to organic electronic devices. At every interface, both organic/organic and organic/inorganic, the potential to utilize nanostructuring to control device performance is very high. In this paper, we focus on one example of nanostructuring at the donor/acceptor heterojunction in organic photovoltaics, with the purpose of modifying efficiency by four orders of magnitude. We show that the length of the exciton dissociating interface can be tuned by changing the substrate temperature for small molecule heterojunction photodiodes based on crystalline DIP/C 60 mixtures. Due to the tuneable interface morphology, the performance of such devices can be changed from poor performing planar heterojunctions to higher efficiency ordered nanoscale bulk heterojunction structures. In this way, highly crystalline DIP can be thought of as a natural "bulk" heterojunction.
Journal of Nano Research 01/2011; 14:125. · 0.57 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report an in situ X-ray investigation of the composition dependence on the structural evolution during drying of doctor-bladed blends of poly-(3-hexylthiophene) (P3HT) and [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM). This study enables an observation of the microstructure evolution in real time during blend crystallization. P3HT:PCBM blends with ratios of 1:0.5, 1:0.8, and 1:2 exhibit differing structural evolution during the course of solvent evaporation resulting in a different microstructure of the blends upon solidification. Large excess of PCBM over the eutectic composition impedes the pi-pi packing of P3HT chains and leads to a not yet observed diffraction feature with an associated spacing of 12.6 angstrom, which might originate from a disordered phase of intimate mixed P3HT and PCBM molecules. This work provides a microscopic understanding of the composition dependence of the film formation from solution. The structural results are discussed in relation to the composition dependence of photovoltaic performance previously reported.
[Show abstract][Hide abstract] ABSTRACT: Metal-organic interfaces based on copper-phthalocyanine monolayers are studied in dependence of the metal substrate (Au versus Cu), of its symmetry [hexagonal (111) surfaces versus fourfold (100) surfaces], as well as of the donor or acceptor semiconducting character associated with the nonfluorinated or perfluorinated molecules, respectively. Comparison of the properties of these systematically varied metal-organic interfaces provides new insight into the effect of each of the previously mentioned parameters on the molecule-substrate interactions.
The Journal of Chemical Physics 12/2010; 133(21):214703. · 3.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this study, we report a systematic investigation of the impact of various processing conditions on all features of the bulk polymer microstructure in working P3HT:PCBM solar cells. Unlike previous studies, which usually examine optimized films grown on Si, without an inorganic electrode layer, we directly examined films in working device architectures to understand the impact of the various steps commonly used in device manufacturing. Thermal treatments were observed to strongly impact the performance of devices produced at less than optimal conditions through changes to the bulk crystal structure; however, even when the production conditions are close to the optimal morphology and crystal structure, annealing is still necessary to improve the π-π overlap of adjacent polymer chains and to reduce the interfacial barrier at polymer-electrode interfaces. The annealing step is therefore crucial to yielding high performance through the control of both the bulk and interfacial properties. Our results suggest a new perspective on device manufacturing, showing that it is not necessary to achieve perfection in bulk crystal structure with the first production steps, potentially saving time in the manufacturing process.
Journal of Renewable and Sustainable Energy 09/2010; 2(5). · 1.51 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this combined atomic force microscopy and X-ray diffraction study, we explore the microscopic origin of the scaling properties of the growth of organic−organic heterostructures formed here by F16CoPc and diindenoperylene (DIP) molecules as representative systems for electron and hole transporting materials. We evaluate the influence of the morphological properties of the first organic layer (DIP) on further temporal evolution of the morphology and structure of F16CoPc films. From the derived scaling exponents, we conclude that the morphology evolution is dominated by mound formation. The microscopic origin of such morphology is ascribed to structural changes occurring during the first stages of F16CoPc growth as revealed by grazing incidence X-ray diffraction (GIXD). The microstructure of organic materials should be taken into account, the challenging task of modeling growing surfaces and interfaces of organic materials.
Journal of Physical Chemistry C - J PHYS CHEM C. 07/2010;
[Show abstract][Hide abstract] ABSTRACT: We investigated the evolution of quaterrylene thin films on SiO(2) and on an octadecyltrichlorosilane self-assembled monolayer (OTS-SAM) to examine the impact of film strains on the growth processes and evolving structure. Surface modification by SAMs allowed tailoring of the growth process from a Stranski-Krastanov (SK) mode (layer-plus-island) on the SiO(2) surface to a Frank-van der Merwe mode (layer-by-layer) on the OTS surface. Detailed structural analysis by x-ray diffraction techniques confirmed that the SK mode was driven by lattice strain in the initial wetting layers on the SiO(2) surface. On the other hand, strain-free wetting layers were already formed at the beginning of growth on the OTS surface, thereby suppressing three-dimensional island formation. Moreover, the films on the SiO(2) surface were found to incorporate high microstrain induced by crystal defects such as dislocations and a mosaic structure. In contrast, few crystal defects were present in the films on OTS surface, demonstrating that OTS treatment enables marked improvement of the molecular alignment. These results clearly indicate that the lattice strain induced by the molecular-substrate interaction is essential for controlling the overall growth process.
The Journal of Chemical Physics 07/2010; 133(3):034706. · 3.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The study of molecular heterojunction morphology is often complicated by the presence of a topographically complex substrate. On such substrates, it is difficult to definitively assign a topographic feature to a specific component. We propose a technique, based on the separation of features in reciprocal space (Fourier subtraction), to deconvolute a heterojunction surface into two real space images. The technique has been successfully applied to three classes of systems: (1) where the overlayer features are smaller than those of the substrate, such as with small molecule growth on polymer substrates (DIP/PEDOT:PSS); (2) where the overlayer features are larger than the substrate, such as with a polymer film in contact with a corrugated metal surface (P3HT/Al), and (3) where both the overlayer and substrate features are of the same size. The Fourier subtraction method extends the study of morphology to heterojunctions with realistic substrates, where the complex topography may previously have prevented a basic description of the specific features of each component in a heterojunction film.